Patent Document 1, for example, discloses a gear having a multitude of minute recesses provided at random in the tooth surfaces thereof. Setting is made such that the average area of the minute recesses ranges from 35 to 150 μm2, that the proportion of the minute recesses on the tooth surfaces ranges from 10 to 40%, that the average surface roughness of the tooth surfaces with the minute recesses ranges from Rmax 0.6 to 2.5 μm, and that the parameter Sk value of the surface roughness of the surfaces with the minute recesses is in the range: Sk≦−1.6. As a result, the oil film forming capacity of the surface is improved, and the oil films between the tooth surfaces of gears are formed to a sufficient degree, whereby damage due to defective lubrication such as surface-originated separation of the gear is prevented, and an improvement in terms of durability is achieved.
Patent Document 1: Japanese Utility Model Laid-Open No. 04-56254